linux-kernel - Re: [PATCH v4 05/12] KVM: x86/pmu: Error when user sets the GLOBAL

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Message-ID: <37a18b89-c0c3-4c88-7f07-072573ac0c92@gmail.com>
Date:   Fri, 7 Apr 2023 13:08:59 +0800
From:   Like Xu <like.xu.linux@...il.com>
To:     Sean Christopherson <seanjc@...gle.com>
Cc:     Paolo Bonzini <pbonzini@...hat.com>, kvm@...r.kernel.org,
        linux-kernel@...r.kernel.org
Subject: Re: [PATCH v4 05/12] KVM: x86/pmu: Error when user sets the
 GLOBAL_STATUS reserved bits

On 7/4/2023 7:45 am, Sean Christopherson wrote:
> On Tue, Feb 14, 2023, Like Xu wrote:
>> From: Like Xu <likexu@...cent.com>
>>
>> If the user space sets reserved bits when restoring the MSR_CORE_
>> PERF_GLOBAL_STATUS register, these bits will be accidentally returned
>> when the guest runs a read access to this register, and cannot be cleared
>> up inside the guest, which makes the guest's PMI handler very confused.
> 
> The changelog needs to state what the patch actually does.
> 
>> Signed-off-by: Like Xu <likexu@...cent.com>
>> ---
>>   arch/x86/kvm/vmx/pmu_intel.c | 2 +-
>>   1 file changed, 1 insertion(+), 1 deletion(-)
>>
>> diff --git a/arch/x86/kvm/vmx/pmu_intel.c b/arch/x86/kvm/vmx/pmu_intel.c
>> index 904f832fc55d..aaea25d2cae8 100644
>> --- a/arch/x86/kvm/vmx/pmu_intel.c
>> +++ b/arch/x86/kvm/vmx/pmu_intel.c
>> @@ -397,7 +397,7 @@ static int intel_pmu_set_msr(struct kvm_vcpu *vcpu, struct msr_data *msr_info)
>>   			reprogram_fixed_counters(pmu, data);
>>   		break;
>>   	case MSR_CORE_PERF_GLOBAL_STATUS:
>> -		if (!msr_info->host_initiated)
>> +		if (!msr_info->host_initiated || (data & pmu->global_ovf_ctrl_mask))
> 
> This is wrong.  Bits 60:58 are reserved in IA32_PERF_GLOBAL_OVF_CTRL, but are
> ASCI, CTR_FREEZE, and LBR_FREEZE respectively in MSR_CORE_PERF_GLOBAL_STATUS.

CTR_FREEZE and LBR_FREEZE are only required for the guest CPUID.0AH: EAX[7:0]>3.
PMU support (ASCI bit) for guest SGX isn't supported either.

So for now, reusing pmu->global_ovf_ctrl_mask here is effective enough.

> 
>>   			return 1; /* RO MSR */
>>   
>>   		pmu->global_status = data;
>> -- 
>> 2.39.1
>>